E: Engine Big Bore Comet

[TomB]

Hi,

I have a big bore Bob Dunn Comet, with MkII 105 cams, 9:1 compression, carb is as follows:

LH Amal MkII 2934/301

34mm

200 main

25 pilot (now fitted to carb body)

Cutaway originally 3.5 (now modified, @4/4.5)

Needle 2A1 pos 1 (weakest) originally – now pos 3, mid.

Set up as this it is usually a first kick starter, on a magneto, and goes well. I haven't set the carb up to idle though, as that loses much of the engine braking, hope that helps!

Tom

 

[Bill Thomas]

Morning Tom , Have you put a smaller rear sprocket on ,
To make use of the extra power.
Cheers Bill.

 

[TomB]

Hi Bill, according to my notes I changed from 46 to 48, which seems about right!

 

[Bill Thomas]

Strange !, Standard Comet is 48 ?, But I know some Twin owners like to use 48 ,
Maybe 2 up or touring ?.
I have a 45 for mine , If I ever get it built , I like to keep the revs low.

 

[ericg]

Hi,

I have a big bore Bob Dunn Comet, with MkII 105 cams, 9:1 compression, carb is as follows:

LH Amal MkII 2934/301

34mm

200 main

25 pilot (now fitted to carb body)

Cutaway originally 3.5 (now modified, @4/4.5)

Needle 2A1 pos 1 (weakest) originally – now pos 3, mid.

Set up as this it is usually a first kick starter, on a magneto, and goes well. I haven't set the carb up to idle though, as that loses much of the engine braking, hope that helps!

Tom

How does it go at full throttle?
My special is 90mm bore, TP 105 cams, 8:1 and I use a 1034 concentric, the main jet is 280.
I'm not sure if this is perfect as the engine won't go over 4500 rpm (about 85 mph) because the bike is way over geared I guess.
But the plug always seems ok, light brown down the porcelain.

I use a STD BSA gearbox with the smaller sprocket (17T) and a Norton Commando rear hub. There is no way to fit a smaller rear sprocket but I have found a rare Spitfire (16T) G/B sprocket. I'll fit this sprocket as soon as I find the last gear (mainshaft 2nd 22T) I need to build an RR gearbox.
As it is the bike doesn't like the STD gearbox with its huge step between 2nd and 3rd gear.

 

[timell]

Hi,

I have a big bore Bob Dunn Comet, with MkII 105 cams, 9:1 compression, carb is as follows:

LH Amal MkII 2934/301

34mm

200 main

25 pilot (now fitted to carb body)

Cutaway originally 3.5 (now modified, @4/4.5)

Needle 2A1 pos 1 (weakest) originally – now pos 3, mid.

Set up as this it is usually a first kick starter, on a magneto, and goes well. I haven't set the carb up to idle though, as that loses much of the engine braking, hope that helps!

Tom

Hello Tom,

I'd be interested to know what you have set the ignition timing to?

Thanks,

Tim

 

[Bill Thomas]

Hi,

I have a big bore Bob Dunn Comet, with MkII 105 cams, 9:1 compression, carb is as follows:

LH Amal MkII 2934/301

34mm

200 main

25 pilot (now fitted to carb body)

Cutaway originally 3.5 (now modified, @4/4.5)

Needle 2A1 pos 1 (weakest) originally – now pos 3, mid.

Set up as this it is usually a first kick starter, on a magneto, and goes well. I haven't set the carb up to idle though, as that loses much of the engine braking, hope that helps!

Tom

Just a thought Tom, My chart says 260 main jet for the 34 mm Amal Mk2 !,
I run a 30 mm Mk2 with a 220 main jet,
You may be ok with a 200 , if you don't ride it too hard ?.

 

[TomB]

Hi Bill,

Thanks for your reply on gearing, I assume I have just written that down the wrong way round and went from 48 to 46. Trying to remember the progression but I think I initially tried 45 and found it lacking a bit at the top end, so then went to 46. I still notice a head wind or hills knocking a bit off the speed.
At present the bike is off the road waiting for a dynamo repair, so I'll check the main jet as well, thanks for the comment!

Tom

 

[Bill Thomas]

Morning Tom , The chart I found for Mk2 carb's ,
Are settings for the size of carb' , Rather than Bike type,
I bought a second hand Mk2 with Methanol jets !,
So had to start again, For my Standard 500, Flash spec' Road Comet,
And the settings they gave for mine made it work well straight away,
I was so pleased I bought a second hand pair for my twin, Same settings !,
Maybe only the slides need changing From bike to bike ?, A small amount.

 

[TomB]

Hi Bill, what carb sizes and settings did you use for the twin? And what manifolds? I'm trying to set up a pair of pre-monoblocs on a B Rapide but they aren't cooperating very well. I had thought of running in on 'modern' carbs to loosen up a newly rebuilt motor and get all the other teething issues sorted out before maybe reverting to original,

Tom